Mix flour, sugar, and salt. Cut in cold butter until the mixture has small irregular crumbs, then mix in the egg just until a dough forms. Chill 20 minutes.
Roll the dough and line a 9-inch tart pan. Prick the base, chill 10 minutes, line with parchment and weights, and bake at 375 F (190 C) for 15 minutes. Remove weights and bake 8 to 10 minutes until pale gold. Cool.
Cook the Custard:
Warm milk and butter in a saucepan until steaming. Whisk sugar, eggs, flour, cornstarch, vanilla, and salt in a bowl until smooth.
Slowly whisk half the hot milk into the egg mixture. Return everything to the saucepan and cook over medium-low heat, whisking constantly, for 4 to 6 minutes until thick and smooth. Do not boil hard.
Pass the custard through a fine sieve into the cooled crust and smooth the surface. Dust lightly and unevenly with cinnamon.
Chill:
Cool for 30 minutes, then refrigerate at least 3 hours until set. Slice with a clean knife and serve chilled or cool.
Notes
Constant gentle whisking and straining produce a smooth custard without bubbles or flour lumps.
Keep covered in the refrigerator for up to three days; the crust softens gradually.